1. Field of the Invention
The present invention relates generally to an alkaline storage battery including a nickel-metal hydride battery, a nickel-cadmium battery, or a nickel-zinc battery, a nickel electrode for an alkaline storage battery employed as a positive electrode of such alkaline storage battery, and a method of fabricating the same, and is particularly characterized in that the nickel electrode for an alkaline storage battery formed by filling a nickel hydroxide-based active material into pore of a porous sintered substrate is modified so that the alkaline storage battery employing as its positive electrode the nickel electrode for an alkaline storage battery performs a high discharge capacity even in a case in which the alkaline storage battery is charged under high temperature conditions.
2. Description of the Related Art
An alkaline storage battery including a nickel-metal hydride battery or a nickel-cadmium battery has conventionally employed as its positive electrode a nickel electrode for an alkaline storage battery in which nickel hydroxide is used as an active material.
Conventionally, as the nickel electrode for an alkaline storage battery, a sintered nickel electrode formed by chemically impregnating a porous sintered substrate which is obtained by filling nickel powder into a porous steel sheet and the like as a substrate and sintering said substrate with nickel salt including nickel nitrate, treating the sintered substrate with an alkaline aqueous solution, and filling the nickel hydroxide as the active material into pore of the porous sintered substrate has been used.
The sintered nickel electrode is superior in collecting current and charge and discharge performance at high electric current because collectivity of the sintered substrate is high and close adherence between nickel hydroxide powder as the active material and the sintered substrate is high.
However, there have remained problems that when the alkaline storage battery employing as its positive electrode the above-mentioned ordinary sintered nickel electrode is charged under high temperature conditions, an oxygen overvoltage of the positive electrode is decreased, thus in addition to a charge reactivity in which the nickel hydroxide is oxidized to nickel oxyhydroxide, a side reaction in which an oxygen evolution reaction occurs and hence a sufficient discharge capacity is not attained occurs.
An object of the present invention is to modify a nickel electrode for an alkaline storage battery formed by filling a nickel hydroxide-based active material into pore of a porous sintered substrate.
Another object of the present invention is to prevent an oxygen evolution overvoltage in a positive electrode from decreasing and to attain a sufficient discharge capacity even in a case in which an alkaline storage battery employing as its positive electrode the above-mentioned nickel electrode for an alkaline storage battery is charged under high temperature conditions.
The nickel electrode for an alkaline storage battery according to the present invention is formed by filling the active material into the pore of the porous sintered substrate, and the active material is obtained by adhering niobic acid to a surface of nickel hydroxide particles. The above-mentioned niobic acid is a hydrate of niobium oxide represented by a compositional formula Nb2O5. nH2O.
The alkaline storage battery according to the present invention employs as its positive electrode the above-mentioned nickel electrode for an alkaline storage battery formed by filling the active material which is obtained by adhering the niobic acid to the surface of the nickel hydroxide particles into the pore of the porous sintered substrate.
As the above-mentioned nickel electrode for an alkaline storage battery, when the active material obtained by adhering the niobic acid to the surface of nickel hydroxide particles is used, the oxygen overvoltage of the positive electrode is increased for the effect of the above-mentioned niobic acid. Thus, when the alkaline storage battery employing as its positive electrode the nickel electrode for an alkaline storage battery is charged under high temperature conditions, an oxygen evolution reaction in the positive electrode which is a side reaction is prevented from occurring and hence the high discharge capacity is attained.
In adhering the niobic acid to the surface of the nickel hydroxide particles, when an amount of the niobic acid based on the nickel hydroxide is too small, the oxygen overvoltage of the positive electrode is not increased sufficiently, thus the oxygen evolution reaction occurs during charge under high temperature conditions and hence the high discharge capacity is not attained. On the other hand, when the amount of the niobic acid based on the nickel hydroxide is too large, so much amount of the niobic acid is interposed between the above-mentioned sintered substrate and the nickel hydroxide that collecting current in the positive electrode is decreased, thus utilization efficiency of the active material is decreased, thereby decreasing the discharge capacity. Therefore, in adhering the niobic acid to the surface of the nickel hydroxide particles, a weight ratio of niobium in the niobic acid based on the nickel hydroxide is preferably set in a range of 0.05 to 3 wt %.
The nickel electrode for an alkaline storage battery formed by filling the active material which is obtained by adhering the niobic acid to the surface of the nickel hydroxide particles into the pore of the porous sintered substrate is fabricated, for example, by filling the nickel hydroxide into the pore of the porous sintered substrate as in the ordinary manner, then immersing the sintered substrate thus having the nickel hydroxide filled into an aqueous solution of at least one type of niobium salt selected from the group consisting of niobium chloride, niobium oxychloride, niobium fluoride, and niobium bromide so that the sintered substrate having the nickel hydroxide filled is impregnated with the above-mentioned niobium salt, and finally immersing the sintered substrate impregnated with the niobium salt into an alkaline aqueous solution including sodium hydroxide so that niobium chloride impregnated in the sintered substrate deposits on the surface of the nickel hydroxide particles as the niobic acid.
The amount of the niobic acid to be adhered to the surface of the nickel hydroxide particles is adjusted by changing number of times of the above-mentioned operation for depositing niobic acid on the surface of the nickel hydroxide particles, or by changing time for which the sintered substrate having the nickel hydroxide filled is immersed in the above-mentioned aqueous solution of the niobium salt.
In addition, in the nickel electrode for an alkaline storage battery according to the present invention, at least one type of element selected from the group consisting of cobalt, zinc, cadmium, manganese, and aluminum is preferably incorporated into the above-mentioned nickel hydroxide particles. Because the oxygen overvoltage of the positive electrode is further increased for the effect of the incorporated elements, thus, when the alkaline storage battery is charged under high temperature conditions, the oxygen evolution reaction in the positive electrode is further prevented from occurring, and the high discharge capacity is attained. Especially when at least one type of element selected from cobalt and zinc is incorporated, higher discharge capacity is attained.
In incorporating the above-mentioned elements into the nickel hydroxide particles, when the amount of the elements is too small, the oxygen overvoltage of the positive electrode is not increased sufficiently, thus the discharge capacity after the charge under high temperature conditions is not so increased as mentioned above. On the other hand, when an amount of the elements is too large, the amount of the nickel hydroxide which is the active material is decreased, thus the sufficient discharge capacity is not attained. Therefore, the ratio of the elements to be incorporated into the nickel hydroxide particles based on the nickel hydroxide is preferably set in the range of 0.5 to 10 wt %.
In the nickel electrode for an alkaline storage battery according to the present invention, the hydroxide of at least one type of element selected from the group consisting of calcium, cobalt, yttrium, and ytterbium in addition to the niobic acid are preferably adhered to the surface of the nickel hydroxide particles. Because the oxygen overvoltage of the positive electrode is further increased for the effect of the hydroxide adhered to the surface of nickel hydroxide particles, thus, when the alkaline storage battery is charged under high temperature conditions, the oxygen evolution reaction in the positive electrode is further prevented from occurring and hence the high discharge capacity is attained. Especially when the hydroxide of at least one type of element selected from cobalt and yttrium is adhered, higher discharge capacity is attained.
In adhering the hydroxide of above-mentioned elements to the surface of the nickel hydroxide particles, when the amount of the hydroxide is too small, the oxygen overvoltage of the positive electrode is not increased sufficiently, thus the discharge capacity after the charge under high temperature conditions is not so increased as mentioned above. On the other hand, when the amount of the hydroxide is too large, electron conductivity of the positive electrode is decreased, thus the sufficient discharge capacity is not attained. Therefore, the ratio of the elements in the above-mentioned hydroxide to be adhered to the surface of the nickel hydroxide particles based on the nickel hydroxide is preferably set in the range of 0.5 to 5 wt %.
Various ways can be employed to adhere the hydroxide of above-mentioned elements in addition to niobic acid to the surface of the nickel hydroxide particles. Examples of such ways include adhering niobic acid and the hydroxide of each of the above-mentioned elements respectively, and adhering a mixture of niobic acid and the hydroxide of each of the above-mentioned elements.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.